The present invention relates to systems and methods for engine control during mode transitions of direct injection stratified charge (DISC) engines.
Operating modes for direct injection stratified charge (DISC) engines may include a homogeneous mode in which the combustion chambers contain a substantially homogeneous mixture of air and fuel, and a stratified mode in which the combustion chambers contain stratified layers of different air/fuel mixtures. Stratified mode generally includes strata containing a stoichiometric air/fuel mixture nearer the spark plug with lower strata containing progressively leaner air/fuel mixtures.
Typically, there is a first range of air/fuel ratios within which stable combustion can be achieved in the stratified mode, such as between 25:1 and 40:1, and a second range in which stable combustion can be achieved in the homogeneous mode, such as between 12:1 and 20:1. As such, there is typically a significant gap between the leanest air/fuel ratio of the homogeneous mode (20 in this example), and the richest air/fuel ratio of the stratified mode (25 in this example). This gap poses a number of challenges in selecting an appropriate operating mode and controlling the engine during transitions between operating modes. For example, careful control of engine operation is necessary to deliver the demanded torque without adversely affecting driveability when switching from the stratified to the homogeneous mode or vice versa.
Methods for using an electronic throttle in combination with fuel injection and spark timing to achieve fast and smooth mode transition in DISC engines without valve timing control are known. However, use of conventional strategies on DISC engines with valve timing control, such as variable cam timing (VCT)applications, may result in reduced fuel economy and deteriorated driveability.
For example, deteriorations in driveability are often associated with changing operating modes between the homogeneous and stratified modes due to the gap between acceptable air/fuel ratios for the two modes of operation in a DISC engine. Because the air charge can not be changed instantaneously, a step change in the fueling rate is necessary at the time instant of the mode switch to satisfy the air/fuel ratio constraints. However, friction torque and the pumping losses do not change instantaneously at the switch point. Consequently, a jump in the fueling rate may lead to a jump in the engine torque with resulting objectionable driveability.
To mitigate the effect of any torque disturbance during mode transitions, some conventional strategies reduce or retard spark timing. However, retarding the spark timing from its optimal setting will typically cause fuel economy penalties and increase feedgas emissions. Furthermore, depending on the engine operating conditions, the spark timing authority may not be sufficient to reduce the torque disturbance to an acceptable level. Thus, mode switching strategies for DISC engines having valve timing control that improve driveability and enhance engine performance are needed.
One object of the present invention is to provide a system and method of coordinated control of fueling rate, spark timing, electronic throttle, and valve timing in DISC engines during mode transitions to improve engine performance.
In carrying out the above object and other objects, advantages, and features of the present invention, a system and method for controlling a direct injection stratified charge engine having an electronically controlled throttle and controllable valve timing during transitions between a current operating mode and a desired operating mode to reduce variations in engine torque from a desired engine torque include moving the throttle to a position associated with the desired operating mode, adjusting fuel flow to substantially maintain the desired engine torque as air/fuel ratio approaches an air/fuel ratio gap between acceptable air/fuel ratios for the current operating mode and acceptable air/fuel ratios for the desired operating mode, and changing valve timing to valve timing associated with the desired operating mode to modify cylinder air charge while changing fuel injection timing associated with the current operating mode to fuel injection timing associated with the desired operating mode as the air/fuel ratio traverses the gap.
The present invention provides a number of advantages. For example, by using valve timing in coordination with an electronic throttle to modulate the intake air to traverse the acceptable air/fuel ratios gap, the discontinuous fuel step and spark retard which are otherwise necessary for torque management can be substantially reduced or eliminated. The present invention can be used for various valve timing control strategies including intake only, dual equal, or dual independent variable cam timing applications, among others. Furthermore, the combination of valve timing control with the stratified charge engine technology allows additional freedom in improving emission constrained fuel economy.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.